Devices for measuring the flow rates of powdery and granular materials as they flow through a gravity flow system comprised of chutes are well known in the art. The inventor herein is either the sole or co-inventor of several such prior art devices as shown in U.S. Pat. Nos. 4,069,709; 4,538,471; 4,543,835; and 4,550,619, the disclosures of which are incorporated herein by reference. The inventor is also aware of other patents for such devices, one of which is U.S. Pat. No. 4,440,029. These patents generally disclose flow rate sensors which include a plate which is placed in the flow path of the material, the plate being supported in some manner from the chute with a force transducer of some sort mounted to the chute or other support and coupled to the plate to detect the force of the material as it impacts the plate. This detected force is then used as a measure for the flow rate. Although many of the plates in the prior art are mounted at an incline with the material dropping vertically onto the plate, at least one as shown in U.S. Pat. No. 4,440,029 is vertically mounted with the material flowing at an angle into the plate.
Because of the wide variance in the size and weight of the plate, there are many different kinds of plate suspension systems utilized in the prior art. These vary in complexity from a single strip spring support for hanging the vertical plate of U.S. Pat. No. 4,440,029 to the leaf springs as shown in the inventor's prior U.S. Pat. No. 4,550,619. However, the universal approach was to support the plate from the chute or other surrounding stationary structure and to do so in as friction free a manner as possible.
In the prior art, it was thought that supporting the plate from the chute or other surrounding stationary structure would help to improve the accuracy of the measurement made by the force transducer by minimizing the effect of the plate mass on the force transducer measurement. Ideally, it was thought that elimination of this effect would result in a force reading based solely on the mass of the material which impacted the plate. Therefore, a significant amount of inventive effort was spent in devising various suspensions which attempted to achieve a frictionless support for the plate, with the force transducer being coupled directly to the plate but not serving in any manner to support the weight of the plate. Although many different approaches were taken in the prior art, inherent limitations in creating a frictionless support, and maintaining its adjustment as it operated, were a constant source of error in the flow rate reading.
It is further noted that, to the best of the inventor's knowledge and experience, little or no effort was made to control the temperature at which the flow rate sensor operated. For a large number of applications, the flow rate sensor would be mounted in an environment which did not have a controlled temperature such that a temperature differential of 40.degree. F. or even more could be experienced between a mid-afternoon and a middle of the night run of material. While the force transducer which was typically used in a flow rate device had a fairly accurate output, and that output would be subject to only a minimal variation over what may otherwise be considered to be a wide operating temperature range, for those applications requiring greater accuracy the temperature variation did introduce a significant error. This is caused by several factors.
First of all, it is not uncommon for a force transducer to be utilized in a flow rate sensor which operated at values much less than the full scale reading for the force transducer. Typically, the offset error due to temperature was expressed as a small percentage of the full scale reading. While this error may not be significant if the force transducer is being utilized at its full scale reading, it becomes quite significant for those applications and flow rates for which the force transducer operates at half or less of its full scale reading, as is typical. Thus, for low flow rates, the inaccuracies introduced by the temperature factor become significant.
Still another aspect of the temperature offset is the fact that the offset varies as the temperature varies, although that variation is generally within the rated range of the device. For example, the temperature offset introduced by operating the force transducer at 30.degree. F. can and usually does differ from the offset introduced by operating the force transducer at 70.degree. F. Therefore, the flow rate of the device can be calibrated while the ambient temperature is at 30.degree. F. to eliminate the temperature offset, but this calibration is lost when the temperature changes from that 30.degree. F. to some other temperature. Thus, an operator may calibrate a flow rate sensor at one temperature, thereby believing that he has eliminated the all extraneous effects from the device, and the device may then shift out of calibration merely through a change in the ambient temperature. Recalibration at the second ambient temperature may temporarily eliminate the temperature offset again, but a return to the previous ambient temperature reintroduces that temperature offset. As might be expected, this can be a frustrating experience for an operator who is not aware of or does not recognize that the flow rate device is designed to utilize a force transducer at only a small portion of its full scale reading such that the temperature offset represents a significant error for low flow rates.
To solve these and other problems of the prior art, the inventor herein has taken a different approach by mounting and suspending the sensing plate directly from the force transducer, there being no other coupling or supporting mechanism connecting the sensing plate to the surrounding chute or other support structure. In effect, the inventor has cantilevered the plate from the force transducer into the chute and in the path of the stream of flowing material. By doing so, the suspension system for the plate which, in the prior art, coupled it to the chute or other support has been completely eliminated, along with the inherent mechanical limitations necessarily present in those suspensions.
More specifically, a pair of beam members cantilever mount the plate directly from the force transducer. These beam members are each generally comprised of a threaded rod which is screwed into the force transducer at one end and coupled to the plate at the other end with a nut. A tube, which could be a length of conduit, surrounds the threaded rod and is compressed between the plate and the force transducer by the force of the nut being tightened onto the rod. This tube provides additional strength and stability. The plate itself is comprised of a pair of pans which are nested together and attached along their edges such that the inner pan is connected to the beam members, and the outer pan is completely flat and smooth. It is this outer pan which is presented to the stream of granular material and acts as a replaceable wear surface. A collar member surrounds one of the beam members and includes several adjusting screws which permit adjustment of clearance between the beam member and the collar. The collar serves as a mechanical limit or stop to prevent excessive damage to the unit should the plate or beam members go out of alignment, or upon failure of the force transducer. The force transducer is mounted in an enclosure, with the beam members extending through the sidewall of the enclosure. To seal the enclosure from dust and other environmental conditions, a pair of dust seals are mounted to the enclosure and the tubes, the dust seals having a bellow-like construction to permit unrestricted reciprocating movement between the beam members and the enclosure as the dry flow sensor operates. The enclosure is mounted to the chute by brackets, so that there is a rigid connection between the force transducer, its enclosure, and the chute.
In order to eliminate the temperature offset problems, in a second embodiment the inventor has enclosed the force transducer in an insulated enclosure, and mounted it directly to the cold plate of a solid state thermoelectric heat pump. A temperature sensor is recess mounted in the force transducer and a control utilizing a computer and a set of relays senses the temperature of the force transducer, compares it with a desired temperature as input by an operator, selects either heat or cool by switching the proper relays, and then powers the thermoelectric cooler with a variable voltage to select just the correct amount of heating or cooling required to bring the temperature of the force transducer back to its desired level.
As a further feature of the dry flow sensor of the present invention, a second force transducer may be mounted in the enclosure and loaded with a counterweight substantially equal to the dynamic load of the first force transducer. Thus, the output of the second force transducer is representative of the vibrational forces in the chute which are also induced in the first force transducer. Although in the prior art mention is made of compensating for vibrational forces, with other designs utilizing plates suspended directly from the chute, these vibrational forces were induced through the plate suspension as well as the force transducer mounting, with possibly some interaction between them. Therefore, with prior art designs it was not readily discernible how to accurately reproduce these vibrational forces, and compensate for them due to the use of multiple suspensions for various portions of the dry flow sensor. However, in the present design wherein the plate is supported directly from the force transducer with no other connection being made to the surrounding structure, the effect of these vibrational forces may be very accurately reproduced by hanging a dummy load substantially equal to the plate and beam support members directly from a second force transducer. The output of these two force transducers may then be summed to subtract out the inaccuracies in the force measurement caused by forces other than the impacting stream of material against the plate. Of course, the dummy load can be scaled down and the output may be multiplied to determine the correction factor or, the second force transducer may even be outfitted with a second plate and beam suspension.
While some of the principle advantages and features of the present invention have been described above, a more thorough understanding may be gained by referring to the drawings and description of the preferred embodiments which follow.